The plaintiffs in the Zantac MDL faced a challenge that is common in products liability cases: How to handle evolving science and conflicting epidemiological data. Predictably, the plaintiffs hired experts to weave favorable epidemiological data into opinions supporting their allegation that Zantac, a heartburn medication also known as ranitidine, had the potential to degrade into a known carcinogen, NDMA. But when the U.S. District Court for the Southern District of Florida found that these experts had not adequately addressed countervailing epidemiological data—among other shortcomings, as detailed in our recent posts on other aspects of the Zantac decision—it excluded their opinions and granted summary judgment against the more-than-2,450 plaintiffs because they had no admissible evidence of general causation.1 The Court's thorough and well-reasoned analysis illustrates that, while cherry-picking data may seem to give plaintiffs an advantage in the short term, it can ultimately open a path to summary judgment for watchful defendants in pharmaceutical cases.

At the forefront of the Court's analysis was an appreciation of the evolving state of the science during the litigation. The initial complaints in the Zantac litigation relied heavily on scientific data from a testing laboratory's citizen petition.2 But the Court noted that FDA later found the testing underpinning the petition to be invalid.3 For instance, a study overseen by Stanford University and initially relied on heavily by the plaintiffs was subsequently retracted by its authors.4 Moreover, none of the many epidemiological studies focusing on ranitidine conducted in 2020 and 2021, after the voluntary recall of ranitidine products, found an association or a causal link between ranitidine and cancer.5

This conflict in the science set up a dilemma for the plaintiffs, who found their theory of the case at odds with the more recent epidemiological studies. Thus, the plaintiffs chose two distinct approaches to prove their case: (1) retaining an expert to test ranitidine for NDMA and (2) introducing the opinions of "epidemiologists who based their opinions, not on the conclusions of any ranitidine-based study author, but instead (for the most part) upon the raw data found in studies that analyzed NDMA-rich food and NDMA-rich air."6

The Court rejected the plaintiffs' experts' experiments and analysis as unreliable under Federal Rule of Evidence 702 and Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993).7 Under that rubric, a court evaluates an expert opinion's "qualification, reliability, and helpfulness."8 While many lessons may be drawn from the Court's application of these criteria to each of the plaintiffs' experts, a key lesson—the need to reconcile conflicting science—is highlighted by the Court's determination that the opinions of plaintiffs' two epidemiologists were unreliable because they did not adequately address competing scientific data.

The Court excluded the opinion of plaintiffs' first epidemiologist that, to a "reasonable degree of medical and scientific certainty, use of ranitidine can cause cancer."9 It concluded, among other things, that her "analysis in this litigation departs from conventional scientific standards" and that "a plaintiff's expert must address epidemiological evidence that is inconsistent with his or her causation opinions."10 Courts have discretion to exclude an expert's testimony when his or her conclusions "are anomalous compared to the conclusions of the other scientists in the field, and the expert has not provided reasons for the anomaly."11 Thus, the Court found plaintiffs' epidemiologist's "effective failure to reconcile her opinion with outside studies to be strongly indicative of an unreliable methodology, and a factor that weighs strongly in favor of exclusion."12

The Court had similar criticisms of the opinions of plaintiffs' other epidemiologist, that "ranitidine causes cancer in humans," and that "the scientific community generally recognizes the toxicity and carcinogenicity of NDMA."13 In examining the bases for her opinion, the Court found that plaintiff's second epidemiologist "all but ignores the study authors' conclusions in the studies she relies upon, the majority of which contradict her opinion and, at the very least, do not support her opinions."14 Further, the Court rejected as unreliable her "[s]elect[ing] data from studies that fit with her ultimate opinion while simultaneously ignoring data in the very same studies that do not fit with her opinions." The Court weighed whether plaintiffs' epidemiologist had employed "objective, scientific-based criteria, or if she instead select[ed] her inputs based upon a results-oriented, conclusion-driven methodology."15 The Court found that, when viewed in totality, the "conclusion-oriented process" she employed, as evidenced in part by her selective use of only supportive epidemiological data, "weighs strongly in favor of exclusion."16

Both these analyses in the Zantac decision underscore that, where competing scientific studies reach different conclusions, an expert must engage the epidemiological data that cuts against his or her opinion and rigorously explain why that data is less reliable or should be given less weight than the data supporting the proffered conclusions. Defense counsel in products liability and mass tort proceedings should closely scrutinize the body of epidemiological literature plaintiffs' experts rely on and determine whether those experts have inappropriately ignored contrary studies that may justify exclusion of their opinions.


1. In re Zantac (Ranitidine) Prod. Liab. Litig., No. 20-MD-2924, ---F. Supp. 3d---, 2022 WL 17480906 (S.D. Fla. Dec. 6, 2022).

2. Id. at *2–3.

3. Id. at *3. FDA also subsequently sent a letter to the testing laboratory, Valisure, noting a number of "methodological deficiencies" in the company's testing processes. ee FDA Dec. 5, 2022 Correspondence from FDA to Valisure, available at (last accessed Feb. 17, 2023).

4. Id.

5. Id.

6. Id.

7. Id.

8. United States v. Frazier, 387 F.3d 1244, 1260 (11th Cir. 2004).

9. In re Zantac at *8.

10. Id. at *129.

11. Id. (emphasis added); see also Norris v. Baxter Healthcare Corp., 397 F.3d 878, 882 (10th Cir. 2005) ("We are simply holding that where there is a large body of contrary epidemiological evidence, it is necessary to at least address it with evidence that is based on medically reliable and scientifically valid methodology."); cf. Kuhn v. Wyeth, Inc., 686 F.3d 618, 633 (8th Cir. 2012).

12. In re Zantac at *129.

13. Id. at *8.

14. Id. at *139.

15. Id.

16. Id.

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